Switch Expressions

TL;DR

• Switch Expressions (finalized in Java 14) transform the traditional switch statement from a control flow structure into an expression that evaluates to a value.
• They introduce the arrow syntax -> which eliminates the need for break statements.
• They allow multiple comma-separated labels per case.

Concept

The old C-style switch statement in Java was notoriously error-prone. If you forgot a break; statement, execution would “fall through” into the next case, causing terrible bugs. Additionally, using a switch to assign a value to a variable was incredibly clunky.

Switch Expressions fix both issues.

1. Expressions: The switch block can now directly return a value (like a ternary operator), allowing you to assign the result directly to a variable: int size = switch(day) { ... }.
2. Arrow Syntax: Using case "Monday" -> completely disables fall-through. Only the code immediately to the right of the arrow is executed. No break required!

Examples

public class SwitchDemo {
    
    public enum Day { MONDAY, TUESDAY, FRIDAY, SATURDAY, SUNDAY }

    public static void main(String[] args) {
        Day today = Day.SATURDAY;
        
        // 1. The Modern Switch Expression (Java 14+)
        // Notice it evaluates to a String, which we assign directly.
        String mood = switch (today) {
            // Multiple labels in one case! No fall-through!
            case MONDAY, TUESDAY -> "Sad"; 
            case FRIDAY -> "Excited";
            case SATURDAY, SUNDAY -> "Happy";
        };
        
        System.out.println("Mood: " + mood);

        // 2. The 'yield' keyword
        // If a case is complex and requires multiple lines of code, you use a block {}
        // Inside the block, you use 'yield' (not 'return') to output the value for the switch.
        int wordLength = switch ("Hello") {
            case "Hi" -> 2;
            case "Hello" -> {
                System.out.println("Processing Hello...");
                yield 5; // Outputs 5 to the wordLength variable
            }
            default -> 0; // Exhaustiveness requires a default case!
        };
    }
}

Interview Questions

Q: What does “Exhaustiveness” mean in a Switch Expression?
A: Because a Switch Expression must return a value, the compiler enforces that every possible input has a defined output. If you are switching on an int, you must provide a default case, because it’s impossible to write a case for all 4 billion integers. However, if you are switching on an Enum, and you write a case for every single enum constant, the compiler considers it exhaustive and you do not need a default case!

Q: Can you still use the colon : syntax in modern Java?
A: Yes. You can use the traditional case "A": syntax as a Switch Expression (by using yield instead of break), and you can use the new arrow case "A" -> syntax as a standard Switch Statement (where it doesn’t return a value). However, you cannot mix colons and arrows in the same switch block.